PESHAWAR, Pakistan - A suicide car bomber ordered to stop his vehicle detonated it instead at a road block in northwestern Pakistan on Saturday, killing seven police officers and wounding several others, a senior police official said.

The bombing underscored the challenge facing Pakistan in rooting out al-Qaida, Taliban and other militants who are based in its northwest and believed involved in assaults on U.S. troops across the border in Afghanistan.

The Saturday morning explosion occurred in an area on the outskirts of the main northwestern city Peshawar, senior police official Safwat Ghayur.

The police were checking cars coming through from the Khyber tribal region, a part of Pakistan's semiautonomous tribal belt where military forces have staged offensives to stem militant activity. The road block was also close to the rough-and-tumble Dara Adam Khel area, a major bazaar from weapons.

Ghayur, who confirmed the casualties, said it was unclear whether the security forces were the original target, although many suicide bombers have gone after Pakistani police.

Ghayur said the attacker blew up his vehicle after he was signalled to stop.

Pakistan recently claimed victory in an offensive against militants in Bajur, a nearby tribal region where the military and insurgents have been battling since August. Officials also say they are close to flushing out militants in nearby Mohmand tribal area.

But while the United States has praised those offensives, saying it has helped reduce violence in neighbouring Afghanistan, Pakistan has raised alarm bells in the West by engaging in peace talks with Taliban militants not far away in the northwest's Swat Valley.